Family relationships

Description

Secretion across the inner membrane in some Gram-negative bacteria occurs via the preprotein translocase
pathway. Proteins are produced in the cytoplasm as precursors, and require a chaperone subunit to direct them to
the translocase component [PMID: 2202721]. From there, the mature proteins are either targeted to the outer
membrane, or remain as periplasmic proteins. The translocase protein subunits are encoded on the bacterial
chromosome.

The translocase itself comprises 7 proteins, including a chaperone protein (SecB), an ATPase (SecA), an integral
membrane complex (SecCY, SecE and SecG), and two additional membrane proteins that promote the release of
the mature peptide into the periplasm (SecD and SecF) [PMID: 2202721]. The chaperone protein SecB [PMID: 11336818] is a highly acidic homotetrameric protein that exists as a "dimer of dimers" in the bacterial cytoplasm.
SecB maintains preproteins in an unfolded state after translation, and targets these to the peripheral membrane
protein ATPase SecA for secretion [PMID: 10418149]. SecE, part of the main
SecYEG translocase complex, is ~106 residues in length, and spans the
inner membrane of the Gram-negative bacterial envelope. Together with
SecY and SecG, SecE forms a multimeric channel through which preproteins
are translocated, using both proton motive forces and ATP-driven secretion. The latter is mediated by SecA.

In eukaryotes, the evolutionary related protein sec61-gamma plays a role in protein translocation through the endoplasmic reticulum; it is part of a trimeric complex that also consist of sec61-alpha and beta [PMID: 8107851]. Both secE and sec61-gamma are small proteins of about 60 to 90 amino acids that contain a single transmembrane region at their C-terminal extremity (Escherichia coli secE is an exception, in that it possess an extra N-terminal segment of 60 residues that contains two additional transmembrane domains) [PMID: 9393849].